1. Field of the Invention
The present invention relates to a frequency conversion circuit, a modulation circuit, a polar modulation transmitting circuit, a quadrature modulation transmitting circuit, a communication instrument, and a frequency conversion method and, in particular, to a frequency conversion circuit, a modulation circuit, a polar modulation transmitting circuit, a quadrature modulation transmitting circuit, and a frequency conversion method of direct conversion type, which are used in communication instruments for mobiles such as mobile phones and wireless LAN.
2. Related Art of the Invention
In recent years the mobile phones and the wireless LAN have become widespread at an explosive pace. Transmitting sections of the mobile phones and the wireless LAN which have become widespread in this way use the direct conversion type that frequency-converts directly into a high frequency signal, hereinafter referred to as a “RF signal” by multiplying a base band signal, hereinafter referred to as a “BB signal”, by a local signal, hereinafter referred to as a “LO signal”. A circuit that makes a frequency convert refers to an upconverter, and includes a frequency conversion circuit that is a circuit serving as the core for a function thereof.
FIG. 8 is a block diagram illustrating a configuration of a conventional frequency conversion circuit, as shown in “Noise in RF-CMOS Mixers: A Simple Physical Model, Hooman Darabi and A sad A. abidi. IEEE TRANSACTIONS ON SOLID STATE CIRCUITS, VOL. 35, NO. 1, JANUARY 2000” as an example. In FIG. 8, the conventional frequency conversion circuit consists of a BB signal input terminal pair 11; a transistor 101 of which base is connected with one of the BB signal input terminal pair 11; a transistor 102 of which base is connected with the other of the BB signal input terminal pair 11; transistors 103, 104 each of which emitter is connected with a collector of the transistor 101; transistors 105, 106 each of which emitter is connected with a collector of the transistor 102; a LO signal input terminal pair 12; a RF signal output terminal pair 13; an electric current source I101; and a ground G101.
One of the LO signal input terminal pair 12 is connected with the base of each of the transistors 103, 106, and the other of the LO signal input terminal pair 12 is connected with the base of each of the transistor 104, 105. One of the RF signal output terminal pair 13 is connected with collectors of the transistors 103, 105, while the other of the RF signal output terminal pair 13 is connected with collectors of the transistors 104, 106 respectively. One end of the electric current source I101 of which the other is connected with the ground G101 is connected with each of the emitters of the transistors 101, 102.
Referring to FIG. 9 illustrating a relationship between signals of respective input terminals and frequencies along with FIG. 8, the operation of the conventional frequency conversion circuit is described below.
The result obtained by multiplying a BB signal B1 inputted from the BB signal input terminal pair 11 by a LO signal L1 inputted from the LO signal input terminal pair 12 is outputted as a RF signal R1 from the RF signal output terminal pair 13. The RF signal R1 is outputted as a signal of a sum of the frequency of the LO signal L1 and the BB signal B1 and a difference therebetween. In the upconveter, the frequency of the sum of the RF signals is selected and is transmitted to a power amplifier on the downstream side.
On the same operational principle, a noise N1 around the BB signal frequency inputted from the BB signal input terminal pair 11 and a noise N2 around a frequency twice as large as the LO signal frequency are outputted to the receiving band frequency R1 of the RF signal output terminal pair 13 by multiplying the LO signal L1 inputted from the LO signal input terminal pair 12. Radiation of these noises as they are would disturb other mobile phones or a receiving circuit in the identical terminal at simultaneous sending/receiving, therefore a low pass filter (LPF) or the like is generally connected to a previous stage of the frequency conversion circuit to suppress the noise inputted into the BB signal input terminal pair 11. However, the LPF, designed so as to obtain a low cutoff frequency, can suppress the noise N1 around the BB signal frequency, while cannot suppress the noise N2 around a frequency twice as large as LO signal frequency, because of a parasitic component, therefore an additional suppressing means is required.
As an example of a frequency conversion circuit for suppressing noise around the frequency twice LO signal frequency, a “frequency conversion circuit” as shown in Japanese Patent Laid-Open No. 2000-295043 (FIG. 1, Page 5-8) is disclosed. FIG. 10 is a block diagram illustrating a configuration of a frequency conversion circuit as disclosed in Japanese Patent Laid-Open No. 2000-295043 (FIG. 1, Page 5-8).
The conventional frequency circuit shown in FIG. 10 includes capacitors C104, C105 added between the collector of the transistors 101, 102 and the ground G102. With such a configuration, both of a differential component and a common component of the noise around the frequency twice as large as LO frequency inputted from the BB input terminal pair 11 can be dropped into the ground G102 through the capacitors C104, C105.
However, in the conventional frequency conversion circuit, capacitors are connected to emitters of transistors 103, 104, 105, 106, so that switching speed at mixing is late.
FIG. 11 illustrates a time-varying characteristic 701 of voltage of node VND1 and a time-varying characteristic 702 of current iCP running through the capacitor 104 in FIG. 10. A rectangular waveform theoretical as an LO signal is illustrated by a dotted line for reference in FIG. 10. Even if the rectangular waveform theoretical as the LO signal is inputted, charging and discharging electrons are repeated in the capacitors C104, C105 each time switching is made, therefore the time-varying characteristic 701 of the voltage of node VND1 takes much time to rise or fall, so that instantaneous switching is impossible and, for example, the transistors 103, 104 of a pair turn on at the same time, which causes a problem that flicker noise may occur. The flicker noise, being mixed with a LO signal, is up-converted, thus causing degradation due to receiving band noise.
That is, the conventional frequency conversion circuit has a problem that flicker noise occurs because switching speed is late at mixing.